Page 327 - Analysis and Design of Machine Elements

Rolling Contact Bearings
Another typical failure mode in rolling bearings is excessive plastic deformation. 305
Under large static loads, balls or rollers will slightly indent the rings and cause perma-
nent deformation. The indentations, permanent deformation or local geometrical dis-
continuities, will cause subsequent noise, vibration and heat as rolling elements pass by.
When rolling bearings work in dusty environment and/or are not properly lubricated,
wear is unavoidable. Insufficient lubrication may also cause scuffing, heat and even burn-
ing in high speed bearings. Fretting wear happens even at light loads when bearings
operate with small-amplitude reversing cyclic motion. Wear results in unacceptable
noise, vibration or heat, as well as contamination of lubricant from accumulated wear
debris [11]. Besides, incorrect mounting may cause cage breakage.

11.3 Life Expectancy and Load Carrying Capacities

The size of bearing is selected according to the load carrying capacity, required design
life and reliability. The load carrying capacity of rolling contact bearing depends on the
total contact area and contact stress, or Hertz stress, which is determined by the mag-
nitude of applied load, the modulus of elasticity of materials in contact and the radii of
contacting bodies. It refers to the basic dynamic load rating and basic static load rating,
which are quoted in the manufacturer’s catalogue.

11.3.1 Life Prediction under Constant Loads

The direct consequence of rolling bearings operating at high cyclic contact stress is
fatigue. The sign of fatigue may be minute spalls or pit appearing on any components
of a bearing. Whenever the first visible evidences of fatigue occur, the number of revo-
lutions or hours that the bearing can operate at a uniform speed is termed the bearing
life.
The life of a rolling bearing is a stochastic variable due to differences in material, man-
ufacturing and assembly. Therefore, reliability is introduced in evaluating bearing life.
The rated life is defined as the number of revolutions or hours that 90% of a random
sample of apparently identical bearings will reach or exceed at a rated load and a design
speed before the first evidence of fatigue appears. For a single bearing, the rated life L 10
refers to the life associate with 90% reliability [7].
To measure load carrying capacity of a bearing against pitting fatigue, basic dynamic
load rating C or sometimes just basic load rating, is introduced. It is the constant load
that a bearing can carry for a rated life of one million revolutions with a 90% probability
against surface pitting. For radial bearings, the basic dynamic load ratings are radial
loads, and for thrust bearings they are axial loads.

11.3.1.1 Relations Between Bearing Load and Bearing Life
Despite using high strength and high hardness steels, all bearings have a finite life and
will eventually fail due to fatigue because of high cyclic contact stresses. Obviously, the
life of a bearing will increase with the decrease of load. A load-life curve of a group
of bearings is obtained by recording the applied loads and corresponding revolution
∘
time when pitting starts at a temperature less than 120 under various stable loads, as
shown in Figure 11.11. Based on extensive experimental data, the relationship between